How to re-organise messy uvs on texture image
It won't get any better than that. This mesh is complex with a lot of small details. So either UVs will be stretched either they will be disjointed. In this case, it is usual to paint directly on a mesh and not on the UV islands.





There are some solutions. Paid or not. This particular one is not ready, but it is about Blender: https://www.youtube.com/watch?v=PLUoLQUK3-s Usually this stuff is done manually, which won't pay off in this case. I suggest to leave it as it is.